Arc tube forming process

ABSTRACT

A quartz arc tube forming process providing a one-piece construction having better uniformity, strength and precision than the prior three-piece construction. The vitreous tubing is heated to plasticity and necked down by cone-sections to a short neck at two places close together. The upstanding double-cone portion between the two necks is then heated and stretched in order to reduce it to the same diameter as the two necks without excessive wall thickening. This procedure is repeated on the tubing at intervals corresponding to the arc tube length and the tubing is severed at the stretched portions to release the formed units.

United States Patent [191 Szilagyi [451 July 29,1975

[ ARC TUBE FORMING PROCESS [75] Inventor: Imre Szilagyi, Cleveland, Ohio[73] Assignee: General Electric Company,

Schenectady, N.Y.

22 Filed: Jan. 14,1974

21 Appl.No.:433,044

[52], US. Cl 65/109; 65/110 [51] Int. Cl. C03b 23/00; C03b 23/08 [58]Field of Search 65/108, 109, 110

Primary ExaminerArthur D. Kellogg Attorney, Agent, or FirmErnest W.Legree; Lawrence R. Kempton; Frank L. Neuhauser [57] ABSTRACT A quartzarc tube forming process providing a onepiece construction having betteruniformity, strength and precision than the prior three-piececonstruction. The vitreous tubing is heated to plasticity and neckeddown by cone-sections to a short neck at two places close together. Theupstanding double-cone portion between the two necks is then heated andstretched in order to reduce it to the same diameter as the two neckswithout excessive wall thickening. This procedure is repeated on thetubing at intervals corresponding to the arc tube length and the tubingis severed at the stretched portions to release the formed units.

6 Claims, 8 Drawing Figures ARC TUBE FORMING PROCESS The inventionrelates to aprocess for forming vitreous envelopes or bulbs from tubingand is particularly useful in making so-called one piece quartz arctubes with preformed end chambers.

BACKGROUND OF THE INVENTION High pressure metal vapor arc lamps such asmercury or metal halide lamps generally comprise an inner arc tube madeof quartz which is enclosed within a vitreous outer envelope or jacketprovided with a screw base at one end. The are tube has electrodessealed into its ends and contains an ionizable medium comprising aninert starting gas and mercury in the case of mercury vapor lamps, ormercury and one or more metal halides in the case of metal halide lamps.

The common economical way to make arc tubes uses so-called full pressseals wherein the entire end segment of a piece of quartz tubing iscollapsed and sealed off. This is done by pinching the ends of thequartz tube while in a heat-softened condition between a pair of opposedjaws to press the quartz about a foliated inlead supporting an electrodeon its inner end. The quartz which is viscous at the instant of pinchingassumes a generally rounded shape in the transition zone from the mainbody to the press seal which may be referred to as the end chamber. Theshape or blow-out of the end chambers varies with the type of quartz,the wall thickness, the heat concentration and the nitrogen pressurebuild-up within the arc tube at pressing. Also there is a seam linewhere the pinched walls come together whose depth and shape may varyappreciably from tube to tube.

In the conventional high pressure mercury vapor lamp which operates withits mercury filling all vaporized, the specific shape of the end chamberdoes not appreciably affect performance of electrical characteristicsand there has been little concern over it. However in the new metalhalide lamps containing a quantity of mercury which is substantially allvaporized in operation and a metal halide in excess of the quantityvaporized, the specific shape of the end chamber governs the location ofthe cold spot where the excess collects and thereby critically affectslamp performance and color. Lamps containing a limited quantity ofmercury and an excess of sodium iodide, and this includes the greatmajority of all metal halide lamps sold commercially, are particularlysensitive to end chamber variations in shape or dimensions. Suchvariations affect the end chamber temperature and hence the color of thelamp which is determined by the balance between the mercury vaporpressure and the vapor pressures of the various metal halides.

One expedient which provides a partial solution to the problem is to usea small-neck arc tube. The pinches or press seals then are smaller andnarrower and the dimensional variations along with temperaturevariations in operation are reduced about proportionately. Howeversmall-neck arc tubes up to the present have generally been made in aso-called three-piece construction wherein a larger diameter quartz tubewhich forms the central body portion is necked down at the ends to whichsmaller diameter pieces of tubing are joined. This is a relativelylaborious and expensive process. Also considerable variation in quartzwall :ness :nxurs at the join between the neck portion and the bodyportion and this introduces dimensional and temperature variations inthe finished arc tube.

The object of the invention is to provide 'a new and improved method ofmaking small neck tubular vitreous envelopes suitable for press sealingor pinching into arc tubes or other form of electric lamps.

SUMMARY OF THE INVENTION The vitreous arc tube forming process of myinvention provides a small neck tubular vitreous envelope in a one-piececonstruction having greater strength and uniformity and betterdimensional constancy then the prior three-piece construction.

In accordance with my process. vitreous tubing is necked down at twoplaces to provide twin short small necks with an upstanding portion ofvitreous tubing between them. The upstanding portion is then heated toplasticity and stretched while shrinking down to approximately the samediameter as the twin necks in order to provide a long neck withoutexcessive wall thickness. The foregoing steps are repeated to formanother long neck portion at another place in the tubing. The tubing isthen severed at the two long neck portions resulting in a formed unit orenvelope having narrow necks at both ends.

The process may be carried out while the vitreous tubing is revolving ina glass lathe. Preferably the tubing is necked down by cone sections inthe transition zones or shoulders between the full diameter of thetubing and the reduced diameter of the short cylindrical neck portions.The terms conical and cyclindrical are intended in a descriptive senseand not as exact geometrical definitions. The necks are relatively shortby comparison with the conical sections or shoulders so that excessivebuild-up of wall thickness is avoided. The upstanding portion betweenthe twin necks .is shaped as a double cone. When heated, it tends toshrink into a prolate spheroid but the stretching elongates it into along neck which merges into the original short twin necks at each end.

DESCRIPTION OF DRAWINGS In the drawings wherein like symbols denotecorresponding parts throughout the several views:

FIG. 1 illustrates pictorially a glass blowing lathe provided withburner and forming roller suitable for the practice of the invention.

FIGS. 2 to 8 illustrate successive stages in processing a piece ofquartz tubing into a one piece small neck are tube.

DETAILED DESCRIPTION The processing of quartz tubing into arc tubeenvelopes by my process may be done using conventional glass workingapparatus. By way of example there is shown in FIG. 1 a glass blowinglathe 1 comprising a head stock 2 including chuck 3 and a tail stock 4including chuck 5. The tail stock 4 is movable on the bedplate 6 and thelathe includes conventional drive means for rotating the head and tailchucks synchronously. An oxyhydrogen surface mix block burner 7 isprovided for directing a broad flame on the quartz tubing and is mountedin front of the tubing on a carriage 8 which is slidable on the bedplate6. A shaping roller 9 madeyqr graphite or heat resistant ceramic ismounted for free turning on an arm 10 pivotally attached to the carriages. The roller 9 is located behind the quartz tubin'g and can be broughtinto engagement with the tubing by pressing down on the lever 11 tocause arm to swing forward. The roller 9 is provided with anannularsurface ridge or embossment 12 corresponding in cross section tothe necking down desired to be achieved in the quartz tubing.

In practicing my process,'a long piece of quartz tubing 14 is insertingthrough the headstock and extends into the tail stock. A rotatingcoupling 15 of conventional construction is attached to one end of thequartz tubing, suitably to the left end beyond the head stock, and theopposite end is stoppered (not shown). A blow tube is attached to therotating coupling to permit the glass blower to pressurize the tube toassist in forming the heat-softened quartz.

The sequence of steps in forming a quartz arc tube by my process isillustrated in FIGS. 2 to 8. The burner 7 heats a short region of quartztubing to the softening point and the roller 9 is pressed intoengagement to produce the first necking down at 16. While rolling is aconvenient way of forming a neck, it may be done in other ways, forinstance by molding. The portion of quartz tubing 14 to the right of thefirst necking is eventually cut off and wasted. Therefore the firstnecking 16 is made as close to the tail stock as conveniently possible.The neck is made relatively short so that as the rolling or moldingproceeds, the quartz is stretched as it is formed into the conicalshoulders 17, 18 and does not thicken up excessively at or near thenarrow neck or throat 16.

In the next step, a second necking 19 is produced to the left and closeby the first as indicated in FIG. 3, and this may conveniently be doneby shifting the carriage 8 appropriately to the left before engaging thequartz tubing. When the forming roller 9 engages the quartz tubing toform necking 19, the quartz has already cooled sufficiently to be rigidin the region of necking l6 and this is necessary to avoid distortion.

The two adjacent short necks 16, 19 where the diameter is at a minimum,havean upstanding double cone portion 20 in between. The third step inmy process consists in shrinking the double cone portion to the samediameter as the two adjacent neck portions 16, 19 while at the same timeavoiding excessive wall thickening. This is accomplished by playing anoxyhydrogen flame on the double cone portion 20 and for this purpose Iprefer to use a'single jet large orifice burner. The softening of thequartz is extended barely into the neck region 16, 19. The double coneportion now begins to collapse into a prolate spheroid. It may bedesirable at this time to pressurize the tube slightly in order tocontrol the rate of collapse and the operator may do this by blowinginto the blow tube. As the double cone collapses into a prolatespheroid, the tubing is stretched by moving the tail stock to the rightso that portion 20 passes through a prolate spheroid 2.1 as illustratedin FIG. 4 to a straight walled elongated neck portion 22 as illustratedin FIG. 5, having about double the length of the original tubing.Substantial uniformity of wall thickness within the stretched long neckportion 22 is achieved by controlling the rate of stretching.

The wall thickness in the long neck may readily be made equal to, lessor greater than the original wall thickness ofthe quartz tubing. Themain control factors are the separation of the twin necks 16, 19 whichdetermines the length of the double cone 20, and the extent of stretchinto long neck portion 22. By increasing the separation and/orstretching less, the wall thickness may readily be made several timesthe original wall thickness of the quartz tubing. This permits makingthick neck tubes for sealing heavier current foils when desired.

With the formation of the long neck 22, one side of an arc tube envelope23 has been formed. A similar neck is next formed on the opposite end bypassing through the intermediate stages of forming two short necks 24,25 as indicated in FIG. 6, and then collapsing and stretching the doublecone portion 26 into a long neck 27 as indicated in FIG. 7. Thereafterthe long necks 22 and 27 are severed at the midpoints and a completedarc tube 23 is released as indicated in FIG. 8.

It is a matter of convenience whether a single bulb or several areformed in the quartz tubing 14 before the individual envelopes aresevered. If the individual envelopes are severed as they are formed,less shifting of the carriage 8 is required but rechucking of the quartztube in the head stock and tail stock is required for every bulb.Alternately, a whole series of bulbs may be formed in the tubing andthereafter the individual arc tube envelopes may conveniently bereleased by severing the tubing at the long necks 22, 27 etc. in anymanner convenient. In yet another mode of operation, the quartz tube 14may be shifted to the right through the tail stock as each arc tubeenvelope is completed and the necks snapped off at the point ofemergence to the right of the tail stock.

The foregoing description of a specific technique followed in processingquartz tubing into arc tube envelopes or bulbs is intended by way ofillustrative example of my method only. It will be apparent that themethod lends itself readily to automation but that in so doing, minorchanges in the steps or in the sequence of steps may be desirable.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A process for forming small neck envelopes from vitreous tubingcomprising the steps of:

heat-softening and necking down the vitreous tubing at two places toprovide twin small necks which are short relative to their shoulderswith an upstanding portion of vitreous tubing between them;

allowing the quartz to cool sufficiently to be rigid at least in theshoulders remote from said upstanding portion; then heatsoftening andstretching the upstanding portion while shrinking it down toapproximately the same diameter as the twin necks in order to provide along neck without excessive wall thickness, said stretching resultingfrom mechanical application of tension to the tubing and said shrinkingresulting from application of heat without use of a die; 9

repeating the foregoing steps to form in a similar way another long neckportion without excessive wall thickness at another place in the tubing;

and then severing the tubing at the two long neck portions in order torelease a formed unit having narrow necks at both ends.

2. The process of claim 1 wherein the vitreous tubing is necked downinto twin small necks by heating to plasticity at the selected placesand rolling down.

3. The process of claim 1 wherein the vitreous tubing is necked downinto twin small necks by heating to plasticity and molding at theselected places.

4. The process of claim 1 wherein the vitreous tubing is necked down bycone sections to produce twin small cylindrical necks at the twoselected places with an upstanding double cone portion between themwhich is then heated to plasticity, and stretched as it is shrunk downinto a long neck portion.

5. The process of claim 1 wherein the vitreous tubing is necked down bycone sections to produce twin small neck portions.

i i i i i

1. A PROCESS FOR FORMING SMALL NECK ENVELOPES FROM VITEROUS TUBINGCOMPRISING THE STEPS OF: HEAT-SOFTENING AND NECKING DOWN THE VITEROUSTUBING AT TWO PLACES TO PROVIDE TWIN SMALL NECKS WHICH ARE SHORTREALTIVE TO THEIR SHOULDERS WITH AN UPSTANDING PORTION OF VITEROUSTUBING BETWEEN THEM, ALLOWING THE QUARTZ TO COOL SUFFICIENTLY TO BERIGID AT LEAST IN THE SHOULDERS REMOTE FROM SAID UPSTANDING PORTION,THEN HEAT-SOFTENING AND STRETCHING THE UPSTANDING PORTION WHILESHRINKING IT DOWN TO APPROXIMATELY THE SAME DIAMETER AS THE TWIN NECKSIN ORDER TO PROVIDE A LONG NECK WITHOUT EXCESSIVE WALL THICKNESS, SAIDSTRETCHING RESULTING FROM MECHANICAL APPLICATION OF TENSION TO THETUBING AND SAID SHRINKING RESULTING FROM APPLICATION OF HEAT WITHOUT USEOF A DIE, REPEATING THE FOREGOING STEPS TO FORM IN A SIMILAR WAY ANOTHERLONG NECK PORTION WITHOUT EXCESS WALL THICKNESS AT ANOTHER PLACE IN THETUBING, AND THEN SEVERING THE TUBING AT THE TWO LONG NECK PORTIONS INORDER TO RELEASE A FORMED UNIT HAVING NARROW NECKS AT BOTH ENDS.
 2. Theprocess of claim 1 wherein the vitreous tubing is necked down into twinsmall necks by heating to plasticity at the selected places and rollingdown.
 3. The process of claim 1 wherein the vitreous tubing is neckeddown into twin small necks by heating to plasticity and molding at theselected places.
 4. The process of claim 1 wherein the vitreous tubingis necked down by cone sections to produce twin small cylindrical necksat the two selected places with an upstanding double cone portionbetween them which is then heated to plasticity, and stretched as it isshrunk down into a long neck portion.
 5. The process of claim 1 whereinthe vitreous tubing is necked down by cone sections to produce twinsmall cylindrical necks at the two selected places with an upstandingdouble cone portion between them, and the double cone portion is thenheated to plasticity and stretched as it shrinks through a prolatespheroid into a long neck portion.
 6. The process of claim 5 wherein thelength and extent of stretch of the double cone portion are chosen toresult in substantially constant wall thickness in the neck portions.